Internal pipe bending mandrels are mandrels which are employed within pipes to support the walls of the pipe during bending of the pipe. Such mandrels may be retracted for movement through a pipe and expanded during pipe bending to support the pipe wall at the inner and outer bending radii, so that the pipe will not be distorted during bending. Internal pipe bending mandrels of several types are shown in U.S. Pat. Nos. 3,274,817, 3,747,394, 3,851,519, and 3,964,290. Other forms of internal pipe bending mandrels are shown in U.S. Pat. Nos. 3,109,477, 3,602,031, and 3,043,361.
Internal pipe bending mandrels must exert very high forces on the pipe wall in order to properly support the pipe wall during bending. Pipe used in construction of petroleum pipe lines may be as large as sixty inches in diameter, and the wall thicknesses may be as much as three-fourths inch, or even more. With pipe of large diameter and wall thickness, the bending forces are very high. The apparatuses known in the art, in general, have not been of sufficient force capacity to adequately protect the pipe walls during bending, and in addition have been excessively complicated, heavy, and expensive. The present invention avoids many of the problems encountered with the prior art apparatuses, by increasing the force capacity of internal pipe bending mandrels, and by reducing weight and cost of such apparatuses.